The Spain international links up with Thomas Frank's side after his previous loan spell with Manchester United was cut short, and his deal is not believed to include an option or obligation to make permanent.
The Bees are believed to have first shown an interest in signing Reguilon back in November, and following a protracted process, they eventually succeeded in their efforts to bring him to the Gtech Community Stadium.
Reguilon is understood to have completed his medical with Brentford on Wednesday and arrives to cover for the injured Rico Henry and Aaron Hickey, the former of whom is expected to miss the remainder of the season with a knee problem.
«This is a very good signing for us. We've been talking for a while about how we need a good left-back, and we managed to get a very good left-back. I'm very pleased with the job that Phil Giles and Lee Dykes have done,» Frank told the official Brentford website.
«Sergio is a perfect fit for us: a proven Premier League player in every aspect. He fits what we want. He's very offensive, has a very good cross and a very good left foot.
»He's got the experience we need to complement the rest of the squad and he's a good character. We need him, but I also think he needs us to show how good a footballer he is. I'm convinced that this will be a positive partnership."
Reguilon himself hailed Frank's influence in his mid-season switch to the Gtech Community Stadium, adding: «I'm very happy, I think this is what I need now in this moment in my career.
»I want to meet my team-mates and play with them, and now just enjoy my football on the pitch. Thomas
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